1. Technical Field
The present invention relates to an inner circumference polishing method of disk-shaped substrate such as a glass substrate for magnetic recording medium or the like.
2. Related Art
In recent years, the production of disk substrates as disk-shaped substrates has been activated, under increased demands as recording media. As a magnetic disk substrate as one of the disk substrates, an aluminum substrate and a glass substrate are used widely. The aluminum substrate is characterized by its high processability and low cost, meanwhile the glass substrate is characterized by its excellent strength, surface smoothness, and flatness. In particular, requirements for compact size and high density of disk substrates recently have become extremely high, and the glass substrate of which surface roughness is small and that enables high density has attracted a lot of attention.
Various improvements have been made in methods of manufacturing such magnetic disk substrates. As the related art described in official gazettes, there is an art of polishing the inner circumferential surface of a glass disk including a portion having a hole at the center (for example, refer to Japanese Unexamined Patent Application Publication No. 11-33886, and No. 11-221742).
In the Japanese Unexamined Patent Application Publication No. 11-33886, piled glass disks are set rotatably around the central axis, and a shaft mounted polishing brush having numerous brush bristles around its axis is inserted into the portions having a hole at the center of the piled glass disks. Thereafter, this shaft mounted polishing brush is reciprocated, while the shaft is rotated in the reverse direction to the rotation direction of the piled glass disk, and thereby the inner circumferential surfaces of the piled glass disks are polished.
Further, in the Japanese Unexamined Patent Application Publication No. 11-221742, a polishing method is proposed where glass substrates are soaked in polishing liquid including separate abrasive, and thereby insufficient polishing and polishing failure due to liquid shortage are prevented. Furthermore, in the Japanese Unexamined Patent Application Publication No. 11-221742, an art is disclosed where brush bristles implanted spirally on rotation axis are rotated to polish the surfaces, and fresh polishing liquid is always circulated and supplied to the surfaces to be polished, and thereby polishing efficiency, reproducibility and precision are increased.
FIGS. 10A and 10B are diagrams for explaining a method of polishing the inner circumference of disk-shaped substrates according to the related art (including the methods of the Japanese Unexamined Patent Application Publication No. 11-33886, and No. 11-221742). In FIGS. 10A and 10B, a brush shaft is inserted into portions having a hole at the center of piled disk-shaped substrates (piled workpieces), and a driving source is placed in the upper direction of the brush shaft (upper shaft direction) so as to rotate the brush shaft. Herein, FIG. 10A shows an inner circumference polishing method where there is no support at the lower shaft of the brush shaft, and the brush shaft rotates in the state where the lower shaft is free. Meanwhile, FIG. 10B shows an inner circumference polishing method where the brush shaft rotates in the state where a support such as a bearing or the like is provided at the lower shaft of the brush shaft.
In this conventional method, in the case of one side support as shown in FIG. 10A, as shown in the right figure of FIG. 10A, the other side that is not supported (non fixed side) swings largely, and the inner diameters of disks at the one side to which the brush swings largely are scraped largely. Further, with the support of the lower shaft as shown in FIG. 10B, the shaft deviates from the support portion of the lower shaft (for example, a lift), and as shown in the right figure of FIG. 10B, the swing of the central portion becomes large, and the inner diameters of disks at the central portion are scraped largely. In particular, when the brush is rotated at a high speed, these swing phenomena become conspicuous. Further, as diameters of disks become small, the diameters of inner circumferential surfaces to be processed become small. Accordingly, when the decrease in the diameter of the shaft is required, the rigidity of the brush shaft becomes low, and the swing at one side (non fixed side) as shown in the right figure of FIG. 10A and the swing at the central portion as shown in the right figure of FIG. 10B become large. As a result, preferable inner circumference polishing cannot be attained.
Meanwhile, there are other related arts with regard to the solution of these problems (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-84828, and No. 2006-7350).
In the Japanese Unexamined Patent Application Publication No. 2000-84828, one end of shaft mounted polishing brush is fixed with a motor, and the shaft end as the other end is supported rotatably by a portion having a hole for supporting a brash shaft. In this supported state, the shaft mounted polishing brush moves up and down together with the portion having the hole for supporting the brush shaft so that the rotation supporting state is continued, while the brush shaft mounted polishing brush reciprocates. Thereby, the swing of shaft end as the other end of the shaft that is not motor fixing side of brush is prevented.
Further, in the Japanese Unexamined Patent Application Publication No. 2006-7350, shaft mounted polishing brush is inserted into portions having a hole at the center of piled glass disks, and is rotated and reciprocated in the state where a load is applied downward to the shaft so as to polish inner circumferential surfaces. It is described that thereby, downward tension is applied to the shaft, and even when the shaft is thin, it is possible to polish respective portions at top, center, bottom of piled glass disks evenly because deflection and a small swing can be prevented.
Furthermore, other polishing method improving these related arts is proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-15450). In the Japanese Unexamined Patent Application Publication No. 2006-15450, there is provided a cylindrical supporting jig that contains and holds plural magnetic disk substrates as objectives of polishing in concentrically piled state. This supporting jig is held rotatably around the shaft via housing. Further, this housing reciprocates in the axial direction of the supporting jig.
Herein, in the structure where tension is simply applied to the other end of shaft and only one end is rotated and driven as mentioned above, for example, distortion phenomenon of shaft increases, and it is difficult to process the inner circumferential surface evenly. In particular, when the shaft core of brush shaft is made flexible, or when it is necessary to make the brush shaft thin for polishing a portion having a small opening hole, for example, in the case where the shaft core of brush is processed by twisting wire and arranging bristle heads spirally, even in the case when tension is applied to the other end, the brush property at polishing process becomes unstable by rotation driving by only one end.
Further, for example, in the case when the piled substrates (the piled workpieces) are arranged vertically, the swing of the piled workpieces becomes large, and it is difficult to polish the portions of the axial direction of the piled workpieces evenly. Furthermore, even in the case when the piled workpieces are arranged horizontally, it is difficult to perform positioning and centering preferably, by only supporting both ends of holder (supporting jig). In particular, when it is necessary to frequently insert and detach the holder, if the holder attachment method is complicated, it takes time and labor for centering and fixation, and much time is required for manufacturing process. Moreover, it is difficult to supply polishing liquid (slurry) to the inner circumferential surfaces of piled workpieces by simply arranging them horizontally, and polishing becomes unstable in some flow states of the polishing liquid. At this moment, for supplying polishing liquid forcibly, for example, pneumatic transportation by a pump is required. In such a case, the size of the apparatus becomes large and mechanism thereof becomes complicated. Further, even with such a supply mechanism, it is not possible to supply polishing liquid uniformly to the entire portions having an opening hole of the piled workpieces, by supplying polishing liquid from only one end of the piled workpieces. As a result, uneven polishing is likely to occur in the portions of the piled direction of piled workpieces.
Herein, in order to sufficiently supply polishing liquid (slurry) to the inner circumferential surfaces of piled workpieces, it is effective to soak the entire substrate holder that holds piled substrates into polishing liquid (slurry), and to apply a sufficient amount of polishing liquid (slurry) onto the entire substrate holder. However, by these methods of the supply of polishing liquid (slurry), polishing liquid (slurry) gets also into a bearing and the like used as brush rotation mechanisms or the like. As a result, it leads to shortened life of the apparatus.
Further, for example, when it is desired to make the inner circumference polishing in the piled workpieces more even, it is effective to restrain uneven polishing by changing polishing conditions, such as changing the relative positions between the polishing brush and the substrate holder and the like. In this case, it is very effective, for example, to invert the piled workpieces in the axial direction. However, in the related art, it is not easy to invert the piled workpieces. As a result, in the related art, there is no choice but to realize preferable polishing with enormous number of man-hour, or to give up adoption of preferable polishing work.
A main object of the present invention is to polish the portion having the opening hole at the center of a disk-shaped substrate in an extremely precise manner.
Another object of the present invention is to precisely polish the portion having the opening hole at the center of the disk-shaped substrate evenly by a simple method.
Further object of the present invention is to provide a method of polishing inner circumference of a disk-shaped substrate for preferably supplying polishing liquid (slurry) to the portion having the opening hole at the center of the disk-shaped substrate, and attaining preferable life of a polishing apparatus.
Furthermore object of the present invention is to restrain the occurrence of a rotation swing of a polishing brush.
Furthermore object of the present invention is to prevent polishing liquid from flowing into, for example, a mechanical portion provided at an end, and to restrain abrasion of a bearing by polishing agent.
Furthermore object of the present invention is to realize further preferable inner circumference polishing of a disk-shaped substrate, for example, even in the case where a polishing brush with a thin shaft easily to bend is used.